Add --enable-developer configure option

Enables features of interest to developers:
 - debug information
 - debug-friendly optimizations
 - treating warnings as errors
Document --enable-developer configure option
Merge branch 'add-developer-config-option'
Fix false positive warning in complete_options()

Said that bool_only can be used uninitialized, which is not true.
Fix false positive warning in query_user_menu()

Said that key can be used uninitialized, which isn't true.
Fix unused local variable warning for array guards
Merge branch 'fixes-for-gcc-4.8.1'